This application claims the benefit of a Japanese Patent Application No. 2002-208797 filed Jul. 17, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus, such as a copying machine, a facsimile apparatus, a printer and a composite apparatus, using a plurality of simultaneously scanning light beams to form a toner image.
In this specification, a composite apparatus refers to an apparatus having composite functions, that is, the functions of two or more apparatuses selected from the copying machine, the facsimile machine and the printer.
2. Description of the Related Art
Conventionally, there is an image forming apparatus, such as a copying machine, a facsimile machine, a printer and a composite apparatus, which is provided with a plurality of semiconductor laser (laser diodes, LDs) as light sources. In such an image forming apparatus, each light beam emitted from the light source makes a scan in a main scan direction by being deflected by a deflecting means which is formed by a rotary polygon mirror having a plurality of mirror surfaces. In addition, scan positions of each of the light beams on a scanning surface are separated in a sub scan direction by a predetermined pitch, so that a plurality of lines can be scanned simultaneously in the main scan direction. According to this recording method, it is possible to improve the recording speed without having to increase the rotational speed of the rotary polygon mirror. However, a satisfactory image cannot be recorded unless write start positions of each of the light beams are correctly aligned.
For example, in the case of the image forming apparatus using two light beams, a synchronization detecting sensor is provided to detect the two light beams. A synchronization detection signal corresponding to each light beam is output from the synchronization detecting sensor when each light beam traverses the synchronization detecting sensor. Hence, generally, an image write start timing of each light beam is determined by the timing of the corresponding synchronization detection signal.
The two light beams traverse the synchronization detecting sensor at a certain time interval therebetween. Accordingly, the synchronization detection signals corresponding to the two light beams have a timing difference corresponding to the certain time interval. No problems will occur if the timing difference between the two light beams detected by the synchronization detecting sensor is identical to the timing difference between the two light beams scanning a photoconductive body. However, if an optical path length from the light source to the synchronization detecting sensor and an optical path length from the light source to the photoconductive body even slightly differ, the timing difference between the two light beams detected by the synchronization detecting sensor and the timing difference between the two light beams scanning the photoconductive body become different, to thereby make the image write start positions of the two light beams different. If the image write start positions of the two light beams are different, the picture quality of the recorded image deteriorates because even a difference on the order of several xcexcm between the image write start positions of the two light beams causes a difference in the image tones recorded thereby.
A Japanese Laid-Open Patent Application No. 2000-292720 proposes a method of suppressing a relative error of light beams in the main scan direction, in an image forming apparatus which forms an image on a recording medium by scanning a rotary photoconductive body by a plurality of light beams deflected by a polygon mirror. According to this proposed method, the relative error between one light beam and another light beam in the main scan direction is measured, and write start timings of the one light beam and the other light beam are electrically corrected depending on the measured relative error.
But in actual practice, the amount of error between the light beams is on the order of one dot or less, and an expensive high-precision measuring device is required to measure such a small amount. As a result, in order to correct the relative error between the plurality of light beams and to suppress deterioration of the picture quality of the toner image which is recorded by simultaneously scanning the photoconductive body by the plurality of light beams, there were problems in that it is necessary to provide the expensive high-precision measuring device to measure the extremely small amount of error between the light beams, and the cost of the image forming apparatus becomes high.
Accordingly, it is a general object of the present invention to provide a novel and useful image forming apparatus in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide an image forming apparatus which can positively and easily correct errors of a plurality of light beams in a main scan direction when forming a toner image by simultaneously scanning a photoconductive body by the plurality of light beams, and form an image having a high picture quality at a low cost.
Still another and more specific object of the present invention is to provide an image forming apparatus is constructed to include an image bearing member which is rotatably supported to bear a toner image, a light beam scanning section to simultaneously scan the image bearing member by a plurality of light beams so as to form an electrostatic latent image, a developing section to develop the electrostatic latent image into the toner image, a transfer section to transfer the toner image onto a recording medium, and an ON start timing adjuster to adjust an ON start timing of one of the plurality of light beams, based on an image tone of each of a plurality of patterns of an image pattern formed by the light beam scanning section. According to the image forming apparatus of the present invention, it is possible to positively and easily correct errors of a plurality of light beams in a main scan direction when forming a toner image by simultaneously scanning an image bearing member, such as a photoconductive drum body, by the plurality of light beams, and form an image having a high picture quality at a low cost. In addition, the present invention may be applied to formation of a monochrome (black-and-white) image and a color image.
A further object of the present invention is to provide the image forming apparatus of the type described above, wherein the image pattern includes first patterns and second patterns; the first patterns being formed by shifting a first light beam in a main scan direction by one dot with respect to a second light beam and repeating an image pattern formed thereby in a sub scan direction, and further repeating an image pattern formed thereby in the main scan direction at intervals of n dots; the second patterns being formed by shifting the first light beam in a direction opposite to the main scan direction by one dot with respect to the second light beam and repeating an image pattern formed thereby in the sub scan direction, and further repeating an image pattern formed thereby in the main scan direction at intervals of n dots, where the main and sub scan directions are approximately perpendicular to each other, and n is greater than or equal to one. According to the image forming apparatus of the present invention, it is possible to easily detect an image tone difference or a latent image potential difference of the first and patterns, so that the ON start timing may be adjusted by the ON start timing adjuster based on the detected image tone difference or latent image potential difference.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.